1. Field of the Invention
The present invention relates to the measurement of distances by laser interferometry in which the interval between the two plates of an interferometer forms the distance to be measured. Since these plates are partial reflectors, they are also called mirrors or reflectors.
2. Description of the Prior Art
In a first known process, the frequency of the laser radiation remains constant and one of the mirrors of the interferometer is moved away from the other mirror along the distance to be measured. The interference fringes which occur during the movement are counted to provide an indication of the distance. This process, which is very precise, is slow and delicate to use, as the device for moving the mirror must conform to very strict tolerances related to the wavelength of the laser radiation, as well as linearity and monotony of the motion. Moreover, such a measurement is very sensitive to turbulence in the ambient atmosphere. This first process is not practically acceptable for distances on the order of one meter or more.
A second process uses a radiation source which emits very short pulses of radiation, and then measures the time which the light takes to propagate over the distance to be measured. Limits, inherent to the time measuring means as well as in the duration of the pulses generated, result in an error on the order of 3 cm in any distance measurement. Precision within this limit is only satisfactory over very great distances, much greater than ten meters.
According to a third process, a modulated electromagnetic wave is applied to an interferometer. The modulation frequency is then adjusted so that it coincides with a resonance frequency of the interferometer. In the case of laser radiation modulated to 100 MHz (wavelength of 3 meters) which is brought to resonate the interferometer with an error on the order of one thousandth of the wavelength, precision is on the order of 3 mm. This process is useful only for relatively great distances (beyond one hundred meters).
According to a process described by MENDRIN et al in U.S. Pat. No. 3,970,389, cyclic variations in the interference pattern from an interferometer are produced by varying the wavelength of the radiation. More precisely, the interferometer produces an adjustable frequency laser beam which is divided between a target mirror and a reference mirror, the distance between which is to be measured. The laser frequency is moved between a starting frequency and a stopping frequency. The number of fringes produced is counted. This count, together with the known starting and stopping frequencies, produces an indication of the desired distance.